test: add unit tests for core game logic utilities

Add 5 new test files covering pure utility functions:
- discipline-math.test.ts (42 tests): stat bonus, mana drain, perk tiers,
  discipline activation/progression, unlocked perks, discipline stats
- formatting.test.ts (35 tests): fmt, fmtDec, formatSpellCost,
  getSpellCostColor, formatStudyTime, formatHour
- floor-utils.test.ts (13 tests): getFloorMaxHP, getFloorElement
- combat-utils.test.ts (37 tests): getElementalBonus, getBoonBonuses,
  getIncursionStrength, canAffordSpellCost, deductSpellCost
- mana-utils.test.ts (36 tests): computeMaxMana, computeRegen,
  computeClickMana, getMeditationBonus, computeEffectiveRegenForDisplay

Total: 163 new tests, all passing. No existing tests broken.

docs/project-structure.txt

@@ -190,8 +190,13 @@ Mana-Loop/
 │       │   │   ├── store-method-tests/
 │       │   │   ├── achievements.test.ts
 │       │   │   ├── bug-fixes.test.ts
+│       │   │   ├── combat-utils.test.ts
 │       │   │   ├── computed-stats.test.ts
+│       │   │   ├── discipline-math.test.ts
 │       │   │   ├── enemy-generator.test.ts
+│       │   │   ├── floor-utils.test.ts
+│       │   │   ├── formatting.test.ts
+│       │   │   ├── mana-utils.test.ts
 │       │   │   ├── regression-fixes.test.ts
 │       │   │   └── spire-utils.test.ts
 │       │   ├── constants/